The invention concerns an osteosynthesis device with a bone screw that has a screw shank head designed as a fork head, where the fork head has a groove running crosswise to the screw axis, and with a corrective rod, in which case the forked head is provided with a means for fixing the corrective rod, and the forked head has an axial opening and a spherical or partly spherical head of the screw shank is moveably inserted into the opening, in which case the opening becomes narrower in its region facing the screw shank and the narrowest cross section of the taper is larger than the diameter of the head of the screw shank.
A large number of osteosynthesis devices are known (DE-Gbm 90 04 240, DE-Gbm 91 04 027, EP-A-346 521, DE-A 39 42 429, EP-A-443 894, EP-A-348 272, EP-A-465 158, EP-A-528 706, EP-A-443 892, DE-A 39 16 198, DE-C 41 10 002, DE 43 16 542 C1). Various bone screws with forked heads are known from these publications, into which a corrective rod can be inserted and fixed. A nonslip anchoring of the corrective rod on the bone screw is thus basically obtained. Only a nonslip and nontwisting fixation of the corrective rod on the bone screw assures an optimal transfer of the tractional and compressive forces on the individual bones to be corrected and fixed as well as the transfer of torque and bending moments.
A good fixation is generally obtained by screwing a fixation screw into the forked bead so that it presses on the inserted corrective rod. However, it has turned out that a bending of the forked head due to the high forces and moments that occur is not precluded, such that the fixation of the corrective rod can loosen. In addition, such a clamping fixation does not offer sufficient security against a twisting of the corrective rod around its own axis.
A bone screw is known from U.S. Pat. Nos. 5,466,237, 5,474,555, 5,360,431 and 5,624,442. The bone screw has a forked head and a screw shank, where the head of the screw shank is supported swivellably in the forked head. An optimal orientation of the forked heads and thus the corrective rod can be obtained in this manner. This is achieved in that the screw shank had a partially spherical head that lies in a sort of ball socket that is provided in the forked head. The ball socket forms a spherical bearing for the head of the screw shank. It turned out, however, that particular care must be taken in the fixing and fastening of the screw shank on the forked head due to the multipart design of the bone screw. If loosening occurs here the goal of stabilizing and fixing the bone is not achieved. Under the circumstances, an immediate operation is necessary.
The invention has as its basis the problem of further developing an osteosynthesis device of the above type so that loosening in the region of fixation of the screw shank on the forked head can be excluded.
This problem is solved according to the invention by designing the narrowing in the form of a truncated hollow cone.
In the osteosynthesis device designed according to the invention the forked head is not, as in the prior art, designed with a ball socket for accommodating the partially spherical head of the screw shank, but has a truncated hollow cone in the accommodation region of the head of the screw shank, in which the head of the screw shank is not merely lying (as is the case in the prior art) but is held clamped so that when the osteosynthesis device is tightened, the head of the screw shank is clamped in this truncated hollow cone profile so that it is fixed by the clamping. On the one hand, the manufacture of such a truncated hollow cone section is considerably simpler than the manufacture of a ball socket, which in any case has to be matched precisely to the diameter of the head of the screw shank. Even minute deviations here result in an unstable support of the screw head. On the other hand, an additional advantage is provided in that, by tightening the means fixing the corrective rod, the screw shank head is pressed into the truncated hollow cone-like narrowing so that it is deformed at least in the bearing region. In the prior art, the partial spherical head is merely pressed on the ball socket, whereby only a frictional closure is obtained. In the design according to the invention a positive locking is achieved besides the frictional closure.
It is provided in a further development that the hollow cone has a cone angle of 2-15xc2x0, in particular, 8xc2x0. A self-locking of the pressed-in screw shank head is achieved with such a cone angle so that even if the means fixing the corrective rod on the forked head is released and the corrective rod removed, the forked head is still securely fixed on the screw shank head.
It is provided in a further refinement that the center of the head of the screw shank lies inside the narrowing. This assures on the one hand that the head of the screw shank is held and pressed in the region of its maximum periphery so that the maximum moments are applied in the region of the pressing due to the maximum lever arm.
One embodiment provides that the opening, on its open end facing the screw shank, broadens by steps. Due to this stepped broadening, a greater clearance is obtained at the screw shank end of the forked head, by which the pivoting angle for the screw shank is enlarged.
To increase the stability of the forked head it is provided that the forked head has an accumulation of material in the region of the hollow cone-like opening. Due to the accumulation of material, plastic deformations of the forked head need hardly or at all be feared and failure due to cracks or the like is excluded. In particular, the accumulation of material is achieved by a great wall thickness. The wall thickness corresponds essentially to the wall thickness of the forked head in the region of the thread for the cap nut.